The present invention relates to a silicon nitride sintered material high in high-temperature strength and superior in low-temperature oxidation resistance, as well as to a process for production of such a sintered material.
Silicon nitride sintered materials have drawn attention for their high high-temperature strength, chemical stability, etc., and various studies have been made on their uses as a material for heat engines such as diesel engine, gas turbine and the like. Since silicon nitride is generally difficult to sinter, a silicon nitride sintered material is produced by mixing a silicon nitride powder with a rare earth element oxide(s) (e.g. Y.sub.2 O.sub.3) as sintering aid to obtain a molding powder, molding the powder into a desired shape, and subjecting the molded material to, for example, firing under gas pressure.
Silicon nitride sintered materials of high high-temperature strength having, as the grain boundary crystal phase(s), apatite structure crystals (hereinafter referred to as H phase) and rare earth element disilicate crystals (hereinafter referred to as S phase), or wollastonite structure crystals (hereinafter referred to as K phase) or cuspidine structure crystals (hereinafter referred to as J phase), or merrilite crystals (hereinafter referred to as M phase) are disclosed in Japanese Patent Application Laid-Open (Kokai) No. 1-56368 or Japanese Patent Application Laid-Open (Kokai) No. 1-61357 or Japanese Patent Application Laid-Open (Kokai) No. 1-61358, respectively.
In general, silicon nitride sintered materials containing Re.sub.2 SiO.sub.5 (hereinafter referred to as L phase) or a S phase as the main crystals of grain boundary have low-temperature oxidation resistance. However, Re.sub.2 SiO.sub.5 is difficult to form as stable main crystals of grain boundary, and a silicon nitride sintered material having an S phase as main crystals of grain boundary is low in high-temperature strength.
In order for a silicon nitride sintered material to show high high-temperature strength, it is preferable that the crystal phase(s) of grain boundary of the sintered material is (are) a crystal phase(s) having a high ratio of rare earth element oxides(s)/SiO.sub.2 and containing nitrogen, such as J or H phase. However, when a silicon nitride sintered material has these crystal phases at the grain boundaries, there occurs selective oxidation of grain boundary phases in air at 800-1,000.degree. C. (this selective oxidation is hereinafter referred to as low-temperature oxidation); as a result, different crystals are formed, resulting in volume expansion and consequent cracking, and the mechanical properties of the silicon nitride sintered material are impaired.
The temperature at which the low-temperature oxidation takes place most violently, is about 900.degree. C. although it varies slightly depending upon the composition of grain boundary phases and the kinds of the crystals constituting said phases.
In order to prevent the low-temperature oxidation, Japanese Patent Application Laid-Open (Kokai) No. 6-227866 discloses a method of heat-treating a silicon nitride sintered material in air to form an oxide layer thereon.
This method, however, has a serious problem in that when the oxide layer is peeled even partially by, for example, foreign objects (Foreign Object Damage (FOD)), low-temperature oxidation begins at the portion where peeling has occurred, which invites cracking.
Further, while very strict dimensional accuracy is required for products such as gas turbine and the like, even if a product of high dimensional accuracy is made by machining of a sintered material, a dimensional error is caused due to a creep when the product is subjected to a heat treatment so as to form an oxide layer.